people are scrambling to debunk an ill-defined phenomenon that's only exists anecdotally... and in the realm of those who ask "qui bono?"
who benefits if "facebook is listening"? what are the incentives?
the question of technical possibility is the first place people will go to... without realizing that if there is a will, there is a way.
Native apps on a phone are not comparable to government spying on networks outside of your control. The attack vector in this case is entirely within the device in possession: the code, and interactions with the OS and hardware.
Entertaining the opposing view: If fb did build this, how is there no hard evidence of it? On iOS, their code is distributed via Apple, so they couldn’t doctor versions for specific users, and any attempt to build self modifying or self deleting mechanisms would be detected sooner or later for similar reasons.